In recent years, in the feedback control field, the H ∞ logic has often been utilized which allows the design of a controller in view of an error between an actual control object and a numerical model of the control object. In the conventional control logic, when designing a control system, a control object model that is represented by the transfer function and the state equation is prepared, and the control system is designed to stabilize the model. At this time, when there is an insignificant error between the actual control object and the model, a controller designed to stabilize the model can also stabilize the actual control object. However, when there is a significant error between the actual control object and the model for some reason, the controller may not stabilize the actual control object.
In the H ∞ logic, even if there is an error between the actual control object and the numerical model for use with the design, when the information as to the error can be obtained, the controller for stabilizing the actual control object can be designed in view of the error. It is said that the H ∞ logic is more likely to give the control specification intuitively in designing the control system, as compared with the conventional control logic. For example, in a case of designing a control system using the conventional control logic, its design specification involved a pole in the closed loop system or a weight matrix of evaluation function. However, the physical meanings of these values were unclear, and it required a lot of trial and error to make the settings.
On the contrary, in the H ∞ logic, the control specification can be defined in accordance with the frequency responses of the closed loop system consisting of the control object and the controller. The H ∞ logic has such an advantage, but is theoretically difficult, and has not been put into practical use in the current situation for the reasons of requiring the considerable knowledge to construct the actual control system, and it being difficult to give the control specification to the objects less treatable in accordance with the frequency responses in the process control and so forth.